An expected double dose of joy turned to tears for tenor titan Luciano Pavarotti yesterday when his girlfriend lost one of the twins she was carrying.

The premature baby, a boy, was stillborn.

New mom Nicoletta Mantovani – in her 31st week of pregnancy – was rushed into surgery at a hospital in Bologna, Italy, for an emergency C-section after tests showed the twin was in distress because he wasn’t getting enough oxygen.

But tragically, surgeons couldn’t deliver him in time to save him.

The child would have been the first son for the opera great, who has three grown daughters by his first wife.

The baby’s twin, a girl, survived the birth.

Asked how Pavarotti was doing after learning of his son’s death, spokeswoman Renata Meroni said, “Without a doubt, it’s one of the worst days of his life.”

The pregnancy had been dangerously complicated from the beginning for longtime Pavarotti gal pal Mantovani, 33, doctors said.

There had been serious doubts about whether the babies’ placenta was getting enough nutrients and oxygen to them, and a hospital test Monday confirmed doctors’ worst fears.

Further tests were conducted, and the emergency operation was then performed.

In an uplifting sign of hope, though, Pavarotti’s newborn daughter was already breathing on her own.

Doctors said that was an especially good sign, considering she weighed in at a teensy 3.8 pounds.

Mantovani also was doing well, the hospital said. Pavarotti, 67, had not left her side, officials said.